Cave-Surveying digest, Vol 1 #64 - 3 msgs

[Garry Petrie]

I can not comment on the frozen field question in lava, but I have
observed station locations where rock displaced the compass something
like 20 or 30 degrees. But compass is mostly about technique, like not
placing a minimag directly on the dial of the compass, etc.

Pat earlier wrote, that if using two compass for fore and back, that
they should be calibrated because, "It is not uncommon for two different
suunto compasses or clino's to differ in their readings sometimes as
much as several degrees." Seems to me, if this statement truly reflects
the common state of instruments, that you had better calibrate your
compass regularly if you only use one!

BTW, please excuse my attack of spam the other day. I felt like a mute
on the Titanic and needed to get something off my chest.

Garry Petrie

> -----Original Message-----
> From: cave-surveying-admin@survex.com 
> [mailto:cave-surveying-admin@survex.com] On Behalf Of 
> cave-surveying-request@survex.com
> Sent: Sunday, January 19, 2003 10:46 PM
> To: cave-surveying@survex.com
> Subject: Cave-Surveying digest, Vol 1 #64 - 3 msgs
> 
> 3. One of the theories is that the magnetic driection 'frozen' in the
lava 
> when it cooled can effect the compass when held close enough to the
rock.
> 
> Garry, have you ever seen the foresite and backsite not agreeing
because of 
> a magnetic bias do to a instrument too close to either the 
> wall/floor/ceiling of the cave or too close to a peice of breakdown?